This invention relates to an improved method for mounting electronic parts on a printed circuit board.
In order to mount electronic parts of relativelY small size on a printed board, it is a well-known practice to tack the parts to the board at predetermined positions using a binder (adhesive), followed bY applying melted solder to the positions to ensure fixation and electric connection between the parts and the board. While a variety of methods are emploYed to carry out the process automatically, a typical method comprises tacking a pluralitY of electronic parts on to a horizontally placed board using an automatic mounting device, turning over the entire board to have the side where the parts are mounted facing down, and bringing the entire board into contact with melted solder. There may be another method which comprises applying a creamy, or pasty, solder to the board in a predetermined pattern using a screen printing technology, tacking the parts to the board, and heating it in an oven to effect soldering in a fluidized state of the solder.
These require that the binder used for tacking the electronic parts should have sufficient stickiness to prevent the parts from dropping out or shifting when the board is turned over and heated. Although various kinds of factors will be possible for this requirement, major factors should include stickiness of the binder, especially when heated, reproducibility and controlability of binder volume, and separability of the binder (lack of stringing when extruded from the dispenser). In addition, it is required that the binder must allow the orientation of the part to easily be corrected when it was inadequate.
Typically in the prior art, a thermosetting binder, such as an epoxy resin, are gently extruded from a nozzle of a dispensing device, e.g. that of a pneumatic type, and retained on the nozzle tip. The binder is then transferred to the printed board at predetermined positions by pressing the tip on the board, where the electronic parts are tacked, and the binder is hardened [Japanese Patent Unscreened Publication (Toku-Kai-Shoh) Nos. 55-110,097 and 61-268,375]. Alternatively, an applicator pin is provided to allow a thermosetting binder from the container to adhere to the tip of the pin, where the binder is transferred to the board for tacking the parts, and then hardened [Japanese Utility Model (Jitsu-Kai-Shoh) No. 57-12,783].
Still another method comprises applying a light-hardening binder, such as an epoxy acrylate or urethane acrylate, to the printed board at predetermined positions, placing the parts on the binder for tacking them, and then hardening the binder by irradiating light, e.g. UV, on it (or placing the parts is preceded by partially hardening the binder through irradiation)[Japanese Patent Screened Publication (Toku-Koh-Shoh) Nos. 63-61,795, and 58-11,114, and Japanese Patent Unscreened Publication (Toku-Kai-Shoh) Nos. 54-104,573, and 54-98,969].
The major problem from which thermosetting binders are suffering is that, because of the momentary fluctuation of their viscosity, the extruding pressure in the dispenser must be adjusted frequently to ensure constant volume of dispensed binder. In addition, once the binder is hardened in the state of tacking the parts on the board, it is impossible to correct the orientation of the parts. Furthermore, in case that a thermosetting binder of increased viscosity is used for the purpose of an elevated securing force for the parts upon mounting on the board, such a binder brings about a problem of stringing during apportionment by the dispenser.
Investigations have been made for methods for the apportionment which will enable to avoid the above mentioned disadvantages involved with the contact-type dispensing device, in which binders are held on the nozzle tip, and then transferred to the board. Among several of designs, a method of non-contact type has been proposed as a conversion from the ink-jet method conventionally employed for printers [Japanese Patent Unscreened Publication (Toku-Kai-Sho) No. 63-228,699]. In this method, binders are sprayed in fine particles caused by the vibration of a piezoelectric device. Because of the droplets' extremely small volume available (approx. 10.sup.-8 cc), a substantially long time is required to attach the necessary volume of binders (approx. 10.sup.-4 cc) the printed board, making this method considered to be unfeasible.
Thus, it is an objective of the present invention to provide an efficient method for mounting electronic parts on a printed board, wherein a constant and controllable volume of binder droplets are ensured, the electronic parts are secured with a sufficient adhesiveness, and an orientational control of the parts is available at any time needed.